The noise generated when a railway vehicle travels is a top-priority issue to be solved for railroad wayside environments. Such noise includes those generated from wheels. Noise generated from a wheel includes wheel rolling noise and so-called squeak noise. The squeak noise is mostly generated when a vehicle passes a curved road having a large curvature. The wheel rolling noise is generated not only when a vehicle passes a curved road but also when it passes a straight road.
The surface of a rail, as well as the surface of a wheel which comes into contact therewith have minute unevenness. As a result, when the wheel rolls on the rail, the wheel and the rail are excited, conceivably causing wheel rolling noise. It is also conceived that the squeak noise is attributable to the vibration of the wheel. Moreover, a peculiar wheel noise occurs in such a way that: when there is a portion that has wave-like wear (hereafter, referred to as “wavy wear”) on a tread part, which is a portion to come into contact with the wheel, in the rail, and if the vehicle passes on such a worn portion, the wheel is severely excited, thus causing noise.
To reduce the noise generated by the vibration of a wheel (hereafter, referred to as “wheel vibration noise”), Patent Literatures 1 and 2 have proposed a soundproof wheel in which a soundproofing device is attached to an inner peripheral side of a rim section of the wheel. Such a soundproof wheel is equipped with a soundproofing device which is composed of a combination of an elastic body section such as rubber and an additional mass section and is attached to an attachment groove formed in an inner peripheral surface of the rim section with a metal attachment.
Such a soundproofing device is considered to act as a dynamic vibration absorber. A dynamic vibration absorber has a resonance frequency, which coincides with a natural frequency of a structure, and can absorb (reduce) the vibration of the structure through resonance. Therefore, by attaching such a soundproofing device to a wheel, it is possible to reduce the noise due to natural vibration of the coinciding frequency. The resonance frequency f of a soundproofing device is given by:f=(½π)×(k/m)1/2  (1)where π is the circular constant, k is a spring constant of the elastic body section, and m is the mass of the additional mass section. Therefore, given a natural frequency of a wheel, it is possible to determine the spring constant k and the mass m of the additional mass section from Formula (1) such that the resonance frequency f of the soundproofing device coincides with the natural frequency.
A wheel normally has multiple natural vibration modes, and natural frequencies corresponding to those natural vibration modes. Therefore, a wheel also has multiple natural frequencies. Patent Literatures 2 and 3 have proposed a soundproof wheel including a plurality of soundproofing devices each having a different resonance frequency, in which the resonance frequency of each sound proofing device coincides with any of the multiple natural frequencies of the wheel. According to these soundproof wheels, it is possible to concurrently reduce noise corresponding to multiple natural frequencies.
Specifically, in the wheel of Patent Literature 2, two soundproofing devices are used in which two types of elastic bodies each having a different elastic constant are used respectively, such that the resonance frequencies of the two soundproofing devices are different from each other. In the wheel of Patent Literature 3, a soundproofing device (a dynamic vibration absorber consisting of an elastic body and an abutment plate) is divided in a circumferential direction of the soundproof wheel, and an adjustment is made such that each resonance frequency of the divided soundproofing devices coincides with any of natural frequencies of the wheel. Such a configuration makes it possible to concurrently absorb noise components corresponding to two or more levels of frequencies. Further, as a result of being divided in the circumferential direction of the soundproof wheel, the soundproofing device disclosed in Patent Literature 3 makes it possible to increase the natural frequencies of the abutment plate to higher frequencies which will not cause any problem as noise.
Among natural vibration modes of a wheel, a primary mode which may cause wheel rolling noise is one in which the rim section vibrates in an in-plane direction and the plate section vibrates in an out-of-plane direction. Such noise due to natural vibrations include at least, 3 levels of frequency components, for example, around 2 kHz, around 3 kHz, and around 4 kHz in a range of 1 kHz to 10 kHz of frequency ranges to which human auditory sense is sensitive.
Meanwhile, when a vehicle is traveling at a high speed, the soundproofing device is subject to various forces, causing a risk that the soundproofing device falls off from the wheel. In the inventions according to Patent Literatures 1 to 3, sufficient measures have not been taken to prevent the soundproofing device from falling off from the wheel.
Patent Literature 4 discloses a soundproofing device including a ring-shape additional mass section, and an elastic body section attached to an outer peripheral portion of the additional mass section, in which the elastic body section is disposed in a groove formed in a rim section.
The soundproof wheel of Patent Literature 4 is configured such that the elastic body section is fixed to a bottom portion of a groove formed in the rim section and to the additional mass section, while the additional mass section is movable with respect to the rim section. Such configuration allows absorption of vibration of the wheel.
In this soundproofing device, since the opening side of the groove is blocked by the additional mass section with respect to the elastic body section, the elastic body section is not likely to fall off from the wheel even if it is separated from the additional mass section. However, in this invention, when the elastic body section deteriorates, the displacement of the additional mass section with respect to the rim section during travelling of the vehicle increases, and the additional mass section may collide with other members, such as a brake disk. Moreover, as a result of that the inner surface of the groove of the rim section and the additional mass section are rubbed against each other, a flaw may occur in the rim section and the additional mass section, or these members may be worn. Further, when the elastic body section is separated completely, there is possibility that additional mass section falls off from the wheel. For this reason, a wheel equipped with the soundproofing device is less durable.